Thermal transfer has become extensively used as a simple printing method. The thermal transfer is a method which comprises the steps of: putting a thermal transfer sheet, comprising a colorant layer provided on one side of a substrate sheet, on top of a thermal transfer image-receiving sheet optionally provided with an image-receptive layer; and image-wise heating the backside of the thermal transfer sheet by heating means such as a thermal head to selectively transfer the colorant contained in the colorant layer to form an image on the thermal transfer image-receiving sheet.
Thermal transfer methods are classified into thermal ink transfer (hot melt-type thermal transfer) and sublimation dye thermal transfer (sublimation-type thermal transfer). The thermal ink transfer is a method for image formation wherein a thermal transfer sheet comprising a substrate sheet, such as a PET film, bearing thereon a hot-melt ink layer, formed of a dispersion of a colorant, such as a pigment, in a binder, such as a hot-melt wax or resin, is provided and energy according to image information is applied to heating means such as a thermal head to transfer the colorant together with the binder onto a thermal transfer image-receiving sheet such as paper or plastic sheets. Images produced by the thermal ink transfer have high density and possesses high sharpness and are suitable for recording binary images of characters or the like.
On the other hand, the sublimation dye thermal transfer is a method for image formation which comprises the steps of: providing a thermal transfer sheet comprising a substrate sheet, such as a PET film, bearing thereon a dye layer formed of a dye, which is mainly thermally transferred by sublimation, dissolved or dispersed in a resin binder; and applying energy according to image information to heating means such as a thermal head to transfer only the dye onto a thermal transfer image-receiving sheet comprising a substrate sheet, such as paper or a plastic, optionally provided with a dye-receptive layer. The sublimation dye thermal transfer can regulate the amount of the dye transferred according to the quantity of energy applied and thus can form gradation images of which the image density has been regulated dot by dot of the thermal head. Further, since the colorant used is a dye, the formed image is transparent, and the reproduction of intermediate colors produced by superimposing different color dyes on top of each other or one another is excellent. Accordingly, high-quality photograph-like full color images can be formed with excellent reproduction of intermediate colors by transferring different color dyes, such as yellow, magenta, cyan, and black, onto a thermal transfer image-receiving sheet, so as to superimpose the color dyes on top of each other or one another, from a thermal transfer sheet of the different colors.
Thermal transfer image-receiving sheets on which images will be formed by these thermal transfer methods have various practical applications. Representative examples of applications include proof sheets, and recording sheets for output images, output plans or designs drawn by CAD/CAM or the like, or images output from a variety of medical analyzers or measuring instruments such as CT scanners and endoscopic cameras. They can also be used as the alternative of instant photographs, and as paper for producing identity certifications, ID cards, credit cards, and other cards on which facial photographs or the like are printed, or for producing synthetic or memorial photographs which are taken at amusement facilities such as recreation parks, game centers, museums, aquariums and the like. The diversification of applications has led to an increasing demand for the thermal transfer of an image on any desired object. A method has been proposed, as one method for meeting this demand, wherein a colorant such as a dye or a pigment is transferred, from a thermal transfer sheet comprising a dye layer or a hot-melt ink layer, onto a receptive layer in an intermediate transfer recording medium comprising the receptive layer separably provided on a substrate to form an image on the receptive layer and, thereafter, the intermediate transfer recording medium is heated to transfer the receptive layer, with the image formed thereon, onto an object (Japanese Patent Laid-Open No. 238791/1987 or the like).
Since the use of the intermediate transfer recording medium permits the receptive layer to be transferred onto an object, this method is preferably used, for example, for objects, onto which a colorant is less likely to be transferred making it impossible to form high-quality images directly on them, and objects which are likely to be fused to the colorant layer at the time of thermal transfer. Further, a method may also be used wherein necessary matters such as signatures or the like are previously written or printed on an object and, thereafter, a transfer portion with an image of characters, a photograph or the like formed thereon is transferred from the intermediate transfer recording medium to prepare a print. Therefore, the intermediate transfer recording medium is preferably used in the preparation of passports or other identity certifications, credit cards/ID cards, or other prints.
Securities, that is, high reliability/safety, which means a high level of difficulty of forgery or alteration, are required of the above-described prints such as passports or credit cards. To make it difficult to perform forgery, alteration or the like by copying, various devices have hitherto been made.
The present applicant has already filed applications regarding an intermediate transfer recording medium having in its transfer portion a hologram pattern, micro-characters, etc., and a print produced by transferring the transfer portion onto an object, for preventing the forgery and alteration of passports, credit cards, or other prints (Japanese Patent Laid-Open Nos. 254844/1999 and 15939/2000). According to these techniques, the transfer of the hologram pattern or micro-characters, provided in the transfer portion, together with the image onto an object makes it difficult to forge and alter images, such as characters or a facial photograph, formed in the print and thus can realize high reliability and safety.